The present application relates to a cathode mix and a nonaqueous electrolyte battery. In particular, the present application relates to a cathode mix including a cathode active material having an olivine crystal structure, and a nonaqueous electrolyte battery including the cathode mix.
In recent year, many portable electronic appliances such as camcorders, cellular telephones, and laptop computers are on the market, and reduction in size and weight of these appliances has been demanded. Therefore, intensive research and development have been carried out to increase the energy density of batteries, particularly secondary batteries, as portable power sources for these electronic appliances. Batteries containing nonaqueous electrolytic solutions, in particular, lithium ion secondary batteries provide higher energy densities than known aqueous electrolytic solution secondary batteries such as lead batteries or nickel cadmium batteries. Therefore, expectations for lithium ion secondary batteries are growing, and the market for them is also remarkably growing.
Lithium ion secondary batteries have lightness and high energy densities, so that are suitable for the use in electric vehicles and hybrid vehicles. Therefore, in recent years, intensive studies have been carried out to increase the size and power of the batteries.
Lithium ion secondary batteries for consumer use are usually composed of lithium cobaltate (LiCoO2) as the cathode active material. However, in consideration of its reserves, lithium cobaltate presents problems of price and supply. Therefore, there will be tendency to use low-cost materials involving low risk of supply shortages.
Under such circumstances, lithium iron phosphate (LiFePO4) composed of iron, which is an abundant and low-cost element, is getting a lot of attention lately. However, in comparison with lithium cobaltate which has been used in related art, lithium iron phosphate gives a lower rate of lithium insertion-elimination reaction, and exhibits high electrical resistance during charge and discharge of the battery. Therefore, sufficient charge and discharge capacity cannot be achieved when charged and discharged at a high current because of the increase of overvoltage.
In order to solve the problem, various studies have been made, and, for example, the following methods (1) to (4) are proposed.
(1) The particle diameter of the active material is decreased, and the specific surface area of the material is increased.
(2) A conductive additive such as carbon is supported on the particle surface of the active material.
(3) Carbon black or fibrous carbon is added during preparation of a cathode mix.
(4) The adhesiveness between component members is improved through the use of a binder having a high binding power.
Specifically, the methods of (1) to (4) are described in the following Japanese Patent Application Laid-Open (JP-A) Nos. 2002-110162, 2001-110414, 2003-36889 and 2005-251554.
(1) JP-A-2002-110162 describes that electron conductivity in the cathode is improved by limiting the primary particle diameter of lithium iron phosphate to 3.1 μm or less, and sufficiently increasing the specific surface area of the cathode active material.
(2) JP-A-2001-110414 and JP-A-2003-36889 describe that the charge and discharge capacity when charged and discharged at a high current is increased by supporting conductive fine particles on the particle surfaces of lithium iron phosphate, and improving the active material.
(3) In order to decrease the electrical resistance of a cathode, powder carbon such as carbon black, flake carbon such as graphite, or fibrous carbon is usually added.
(4) JP-A-2005-251554 describes that the use of a binder having a high binding power improves adhesiveness between the cathode active material and the conductive additive, between the cathode active material and the collector, and between the collector and the conductive additive thereby improving the characteristics during charge and discharge at a high current.